In the prior art, for example, a catalytic layer comprising a complex of carbon supported Pt (carbon supporting Pt as a catalyst) and a fluorocarbon-type ion exchange resin (Nafion®) is known. When a complex of a carbon supported Pt and fluorocarbon-type ion exchange resin is used, there must be a three-phase interface in which a region where electrons flow (electron conductive region) and a region where the ions flow (ion conductive region) and pores for gas diffusion (pores) are all present. Additionally, the catalyst itself must be present at the three-phase interface.
Generally, this type of catalytic layer of the prior art is manufactured by the following method: (1) the electron conductive member, catalytic component, and ion conductive member are mixed so that suitable pores are present; (2) a suitable solvent, is added to form a paste and is coated onto an electrode base and (3) the solvent is dried and removed. Such an example is disclosed in Japanese Laid-Open Patent Publication Number 2000-106203.
Additionally, in recent years, there has been much interest in using a conductive polymer instead of carbon for the catalytic layer of the fuel cell electrode. Conductive polymers have the same electroconductivity as carbon, and conductive polymers are highly porous like carbon. Therefore, there is adequate gas diffusivity.
For example, with the catalytic layer of the cathode disclosed in U.S. Pat. No. 5,346,780, (1) a fiber-like mass of a mixture containing a modifying catalyst (a proton conductive thin film formed on top of a carbon supported Pt) and polytetrafluoroethylene is deposited on carbon paper and sintered to form an electrode substrate; and (2) a thin film in which trifluoro-methanesulfonic acid is retained within a polyaniline network is formed by electrolytic polymerization on top of this electrode substrate (in this case, polyaniline is used as the polymer network and trifluoro-methanesulfonic acid is used as the proton conductive monomer.)
The catalytic layer of the fuel cell electrode disclosed in U.S. Pat. No. 6,117,581 (reference patent 3) is created by the following method: (1) channels are dehydrated by heating zeolite, and after introducing aniline monomers into the channels, this is heated under pressure and polymerized with a suitable catalyst, forming polyaniline in the zeolite channels; (2) catalytic particles (for example Pt) are uniformly dispersed in a mixture of zeolite incorporated with polyaniline and carbon particles, and this is mixed in a solution to make an ink; and (3) this ink is hot pressed onto an electrolyte membrane.
Additionally, a transitional metal (Pt) can be covalently bonded to the hetero atom contained in a conductive polymer (polyaniline). The catalytic layer of the fuel cell electrode disclosed in U.S. Pat. No. 6,479,181 is obtained by the following method: (1) a chloroplatinic acid solution is added to a polyaniline suspension solution and stirred; (2) the polyaniline-PtCl4 complex is separated by centrifugation, (3) Pt is reduced using a reducing agent, and (4) this is dried.
However, the catalytic layers of the fuel cell electrodes of the prior art had the following problems.
Firstly, the catalytic layer must have adequate electroconductivity, ionic conductivity, and gas diffusivity. However, in Japanese Laid-Open Patent Publication No. 2000-106203 for example, the catalytic layer is a mixture of materials each relating to electroconductivity, ionic conductivity, and gas diffusivity, and if the amount of the electroconductive member (carbon) is increased in order to increase electroconductivity, the relative amount of ionic conductive member (fluorine-type ion exchange resin) is reduced, and ionic conductivity is reduced. There are tradeoffs of the different properties.
Additionally, because the site of the electrode reaction in the catalytic layer is at the three phase interface described above, the three phase interface must be formed efficiently. With the catalytic layer of the Japanese Laid-Open Patent Publication No. 2000-106203, for example, even if the three-phase interface is created in the location where the reaction will occur, it is difficult to maintain continuity where the three-phase interface (each phase) is all connected in the catalytic layer. For example, because the mixing method, mixing ratios, and the like influence the properties (electroconductivity, ionic conductivity, and gas diffusivity), the catalytic component could not be dispersed while maintaining the continuity of each phase, and an ideal three phase interface could not always be formed. As a result, in areas where the three-phase interface is not formed, the catalyst does not perform, and the catalyst utilization rate is poor.
Secondly, with the catalytic layer of the prior art (for example Japanese Laid-Open Patent Publication No, 2000-106203), the catalyst is attached to a surface of a support with a large surface area and is electroconductive (a carbon material such as carbon black). This is in order to prevent assembly and aggregation of the catalytic components to each other and to prevent reduction in dispersibility. However, even when the forces that act between Pt molecules are relatively weak, such as an intermolecular force, aggregation of Pt is not adequately prevented. As a result, a catalytic layer with sufficient dispersion of catalyst is not obtained.
With known supports such as carbon black, the number of bonding sites for the catalytic components is sparse. As a result, when there is a high density of catalytic components which is greater than the number of bonding sites, the aggregation and cohesion of catalytic components cannot be prevented. As a result, a catalytic layer having sufficient dispersion of catalyst is not obtained.
Similarly in U.S. Pat. No. 5,346,780, because a polyaniline network thin film in which tri-fluoro methanesulfonic acid is retained is formed on surface of an electrode substrate, it is not possible sufficiently to disperse Pt over the entire catalytic layer.
In U.S. Pat. No. 6,117,581, catalytic particles are uniformly dispersed in a mixture of polyaniline in zeolite and carbon particles, and this is used to form a catalytic layer on surface of an electrolyte membrane by hot press. Therefore the aggregation of catalytic particles is not suppressed, and the catalyst is not sufficiently dispersed over the entire layer. The number of bonding sites for the catalytic particles on carbon and zeolite is limited, and if there is a higher density of catalytic components than the number of sites, the aggregation of catalytic components cannot be suppressed.
In U.S. Pat. No. 6,479,181, a platinum chloride acid is added to a polyaniline suspension solution. After conducting a prescribed treatment, this is reduced, and the transitional metal is covalently bonded to the hetero atom. Therefore, the force acting between the support and the catalytic component is thought to be relatively strong. However, because platinum chloride acid is added later to polyaniline which is a polymerized product, the platinum chloride acid does not adequately enter the polyaniline, and the catalytic component cannot be dispersed at a high density.
As described above, if the catalytic component cannot be dispersed adequately at high densities, the catalyst utilitzation rate is poor. This has a negative impact on the efficiency and output of the fuel cell.